linux-kernel - Re: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu

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Date:	Tue, 5 Jul 2016 11:34:12 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Jan Beulich <JBeulich@...e.com>,
	Joao Martins <joao.m.martins@...cle.com>
Cc:	Andrew Cooper <andrew.cooper3@...rix.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Juergen Gross <JGross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH linux 2/8] xen: introduce xen_vcpu_id mapping

On Thu, Jun 30, 2016 at 03:10:11AM -0600, Jan Beulich wrote:
> >>> On 29.06.16 at 18:27, <andrew.cooper3@...rix.com> wrote:
> > On 29/06/16 17:19, Vitaly Kuznetsov wrote:
> >> To explain better what I'm trying to suggest here please take a look at
> >> the attached patch. If we can guarantee long term that ACPI id always
> >> equals to Xen's idea of vCPU id this is probably the easiest way.
> >>
> >> -- Vitaly
> > 
> > The code in hvmloader which sets up the MADT does:
> > 
> >     for ( i = 0; i < hvm_info->nr_vcpus; i++ )
> >     {
> >         memset(lapic, 0, sizeof(*lapic));
> >         lapic->type    = ACPI_PROCESSOR_LOCAL_APIC;
> >         lapic->length  = sizeof(*lapic);
> >         /* Processor ID must match processor-object IDs in the DSDT. */
> >         lapic->acpi_processor_id = i;
> >         lapic->apic_id = LAPIC_ID(i);
> >         lapic->flags = (test_bit(i, hvm_info->vcpu_online)
> >                         ? ACPI_LOCAL_APIC_ENABLED : 0);
> >         lapic++;
> >     }
> > 
> > So relying on the acpi_processor_id does look to be reliable.  That code
> > hasn't changed since 2007, and that was only a bugfix.  I would go so
> > far as to say it is reasonable for us to guarantee this in the guest ABI.
> 
> In fact - is there any other way a guest could learn the vCPU IDs
> of its CPUs in a reliable way? I don't think so, and hence this de
> facto already is part of the ABI; we should of course spell it out
> somewhere.

CCing Joao.

Joao worked (and I think he posted an RFC patchset?) where this is changed so
that the true hardware topology (core, thread, etc) is exposed. This is obviously
for cases where you want pinning.

I would hesistate to spell this out as an ABI..

P.S.
Which reminds me, Joao, you OK posting the patchset?
> 
> Jan
> 
> 
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